Abstract

A ground-water flow model of the Mississippi River Valley alluvial aquifer in eastern Arkansas, developed in 2003 to simulate the period of 1918-98, was validated with the addition of water-level and water-use data that extended the observation period to 2005. The original model (2003) was calibrated using water-level observations from 1972, 1982, 1992, and 1998, and water-use data through 1997. The original model subsequently was used to simulate water levels from 1999 to 2049 and showed that simulation of continued pumping at the 1997 water-use rate could not be sustained indefinitely without causing dry cells in the model.

After publication of the original ground-water flow model, a total of 3,616 water-level observations from 698 locations measured during the period of 1998 to 2005 became available. Additionally, water-use data were compiled and used for the same period, totaling 290,005 discrete water-use values from 43,440 wells with as many as 39,169 wells pumping in any one year. Total pumping (which is primarily agricultural) for this 8-year period was about 2.3 trillion cubic feet of water and was distributed over approximately 10,340 square miles within the model area.

An updated version of the original ground-water flow model was used to simulate the period of 1998-2005 with the additional water-level and water-use data. Water-level observations for 1998-2005 ranged from 74 to 293 feet above National Geodetic Vertical Datum of 1929 across the model area. The maximum water-level residual (observed minus simulated water-level values) for the 3,616 water-level observations was 52 feet, the minimum water-level residual was 60 feet, the average annual root mean squared error was 8.2 feet, and the annual average absolute residual was 6.0 feet. A correlation coefficient value of 0.96 was calculated for the line of best fit for observed to simulated water levels for the combined 1998-2005 dataset, indicating a good fit to the data and an acceptable validation of the model.

After the validation process was completed, additional ground-water model simulations were run to evaluate the response of the aquifer with the 2005 water-use rate applied through 2049 (scenario 1) and the 2005 water-use rate increased 2 percent annually until 2049 (scenario 2). Scenario 1 resulted in 779 dry cells (779 square miles) by 2049 and scenario 2 resulted in 2,910 dry cells (2,910 square miles) by 2049. In both scenarios, the dry cells are concentrated in the Grand Prairie area and Cache River area west of Crowleys Ridge. However, scenario 2 resulted in dry cells to the east of Crowleys Ridge as well. A simulation applying the 1997 water-use rate contained in the original ground-water flow model resulted in 401 dry cells (401 square miles) in the Grand Prairie and Cache River areas.

First posted March 19, 2009